The formation index of filter paper is one of the most important characteristics used in industrial quality control. Its estimation is often based on subjective comparison chart rating or, more objective, on the power spectrum of the paper structure observed on a transmission light table. It is shown that paper formation can be modeled by means of Gaussian random fields with a well-defined class of correlation functions, and a formation index which is derived from the density of the Bartlett spectrum estimated from image data: the mean of the Bessel transform of the correlation function taken for wave lengths between 2 and 5 mm. Furthermore, it is shown that a considerable variation of the local grammage can be observed also in cases where the the fibers are uniformly and independently scattered in the paper sheet.

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